1. Field of the Invention
This invention relates to a slit exposure shutter for a camera, such as a focal plane shutter, and more particularly to an electromagnetically driven slit exposure shutter which is driven by an electromagnetic force to make an exposure effecting travel and a return travel.
2. Description of the Prior Art
The recent advancement of applications of electronics to cameras has prompted the use of electronic circuits for light measurement, exposure control, information display, etc. However, in most cases, the shutter blades are still arranged to be driven by the force of a spring which is arranged to be charged by a winding-up operation. Particularly, in the case of a focal plane shutter which must be driven by a relatively large driving force, a mechanical driving force is often used in view of the problem that would be presented by the limited capacity of a battery usable within the camera for electrically driving the shutter instead of mechanical driving. The focal plane shutter which is arranged to be operated by a mechanical force in this manner, however, requires the use of a complex mechanism for driving shutter blades and thus is disadvantageous in terms of reduction in size of a camera. It has been, therefore, a general desire to have the shutter of this type arranged to be electromagnetically driven and this desire has come to be responded by an increased number of proposals.
On the other hand, it is general desire that a camera is compact in size for sufficient portability. This requirement necessitates the use of a small battery which does not have sufficient capacity as power source for supplying a power to an electromagnetic device to be incorporated in the camera. As for the electromagnetic device, it is necessary to use a compact device which does not have a sufficiently high efficiency in the electric-to-mechanical energy conversion required. Accordingly, where the shutter blades must be electromagnetically driven instead of mechanical driving, the driving force available naturally becomes smaller than a mechanical driving force.
The present invention is directed to the solution of the above stated problem presented by the conventional electromagnetically driven shutter.